Rigid PVC Injection Molding: Corrosion, Residence Time and a Twenty-Degree Window

Why anyone moulds PVC at all

PVC is cheap, flame retardant by nature, chemically resistant and dimensionally stable. Those four properties make it the default material for pipe fittings, electrical conduit boxes, cable glands, window hardware and a long list of industrial parts.

It is also the most difficult commodity resin to injection mould. The processing window is 20 °C wide, the material releases hydrogen chloride when it overheats, that acid attacks the machine and the tool, and the compound contains plasticisers and stabilisers that migrate and plate out.

A shop that moulds PVC successfully has made specific investments: corrosion-resistant screw and barrel, a tool with stainless or chrome surfaces, good extraction, and a discipline about residence time that other materials do not require.

Table 1 — PVC compound types. phr is parts per hundred of resin; the plasticiser content decides both flexibility and how the material behaves in the barrel.

The thermal problem

PVC melts at 160 to 180 °C and degrades above about 210 °C. That leaves a 30 °C band in theory, and 20 °C in practice once you account for the fact that shear heating adds to the barrel temperature and the melt is hotter than the set point.

Degradation is autocatalytic. A chain breaks and releases HCl; the HCl catalyses further breakdown. The reaction accelerates, and the visible signs follow in order: yellow, then brown, then black. By the time you see black, the barrel is contaminated and the next three hours are cleaning.

The stabiliser package exists to interrupt this. Calcium-zinc and organotin stabilisers scavenge the HCl as it forms. They are consumed in the process, which is why residence time matters: a compound held at temperature for 30 minutes has used up more stabiliser than one held for 5.

This is why PVC moulding has a rule that no other material has: the machine must not be left sitting hot with PVC in the barrel. A lunch break with the barrel at 195 °C is enough to produce a degraded plug at the screw tip that will appear in the first parts after the break.

Corrosion: what it attacks and what to do

Hydrogen chloride in the presence of moisture forms hydrochloric acid, and hydrochloric acid attacks carbon steel, brass and most aluminium alloys. A standard nitrided screw and barrel will show pitting within months of running rigid PVC.

The correct specification is a bimetallic barrel with a corrosion-resistant liner, a screw with a tungsten carbide or Colmonoy hard facing, and a non-return valve in a corrosion-resistant grade. This is not a premium option; it is the minimum for a machine that will run PVC regularly.

Tooling is the same story. Cavities and cores in standard P20 will corrode and lose their polish. Through-hardened stainless, or a hard chrome plate over a suitable base, is the standard. Aluminium tools are acceptable only for prototypes and only with a coating.

Extraction is part of the equipment, not a building afterthought. HCl vapour at the nozzle and at the mould opening must be extracted at source, or it will corrode the machine frame, the electrical panels and everything else in the bay.

Residence time: the limit that is not on the datasheet

The practical limit for rigid PVC at 195 °C is about 10 minutes of residence. Beyond that the stabiliser is depleted and the colour starts to shift. At 200 °C the limit is closer to 6 minutes; at 185 °C it can be 20.

Residence time is calculated from the shot weight and the barrel capacity, not from the cycle time alone. A 20-gram part on a machine with a 500-gram shot capacity has 25 shots of material in the barrel; at a 30-second cycle that is over 12 minutes of residence before anything has even happened.

This is why PVC parts are moulded on small machines. Oversizing the press for tonnage reasons is the most common cause of colour problems in PVC, and it is invisible on a setting sheet.

Table 2 — Residence time limits for rigid PVC. The number falls faster than the temperature rises, which is why the window is effectively 20 °C wide.

Settings that work

Barrel profile for rigid PVC is flat or slightly rising, 175 to 190 °C, with the nozzle at the low end. A hot nozzle is the most common cause of drool and of degradation at the gate.

Injection speed should be moderate. PVC is shear-sensitive and a fast shot through a small gate generates enough shear heating to push the melt past the degradation threshold locally. The result is a brown streak that follows the flow from the gate.

Mould temperature is 30 to 50 °C. Colder gives a better surface but more frozen stress; hotter gives better fusion and a longer cycle. For fittings that must pass a pressure test, hotter is safer.

Packing pressure should be modest — 40 to 60 percent of injection — and packing time short. PVC is nearly amorphous and does not shrink much, so there is little to compensate and over-packing builds stress at the gate.

Table 3 — Rigid PVC injection moulding settings. Every parameter here is chosen to minimise shear heating.

Defects

Table 4 — PVC injection moulding defects. Three of the eight are degradation, which is the defining risk of the material.